You can buy several types of guides that attach to your router so you can create decorative circular grooves on the surfaces of wood projects, or you can make up your own. Besides making grooves of various designs by switching router bits, these jigs can also be used to make circular cut-outs. Instead of cutting only partly into the surface, continue with shallow passes until the cut is all the way through the wood.
Instead of putting a paint smear across the outside of a can to mark the level of paint left inside (which can be messy and obliterate the label), use a rubber band instead. Put it around the can and roll it down to the level of the paint. Just by glancing at the rubber band, which is visible all the way around, you will know how much paint is left.